Such wound stapling instruments which are disclosed in EP-A-040 683. EP-A-142 225, EP-A-284 345, EP-A-244 854, U.S. Pat. No. A-1,945,377 and U.S. Pat. No. A-3,873,016, are used in particular for the closure of skin wounds in operations, and are, to an increasing extent, being used instead of the traditional method of stitching wounds.
The use of wound staples does, however, have certain disadvantages in clinical practice. For a scar to heal well, the healing of the wound should begin at the lowermost layer of the skin. The surgeon performing the stapling operation accordingly stands the edges of the wound slightly upwards, before their being stapled, and then secures such so-called "everting" of the wound edges with a staple seam. Where the implantation of the wound staples is poorly carried out, the everting position may not be maintained, resulting in stepped scars which are disadvantageous, both cosmetically and also with regard to the stability of the scar.
This disadvantage may be attributed on the one hand to the shape of the staple being unfavourable, but on the other hand principally to malfunctioning of the wound stapling instrument. It has accordingly been found that the staples are frequently deformed asymmetrically, although they are stored in the staple magazine in precise symmetrical alignment, the dies for deforming the staple, namely on the anvil and the driver, having no detectable asymmetry.
One reason why such asymmetrical deformation of the staple may occur is because during the deformation or closure of the staple, the lower edge region of the staple, lying on the anvil, is displaced with respect to its center. Another reason is that, the arching of the staple which automatically occurs in the region of its crown draws staple material inwards, because the driver presses onto the staple by way of two legs which are directed downwardly onto the staple on both sides of the center of the staple, whereby the crown of the staple is raised from the anvil thereby producing said arching.
For the above reasons, tractive forces acting on the staple are produced at the two contact points between the staple and the anvil. These tractive forces, which are oppositely directed, firstly occur uniformly to the left and to the right, but only until the tractive force exceeds the static friction force at one or other contact point. The staple will then start to slide on one side in the respective contact zone and will continue to slide, because the coefficient of sliding friction, is always less than the coefficient of static friction. The staple will, therefore, always be drawn more towards one side and will not be implanted symmetrically, especially when said slipping of the staple on the anvil is additionally adversely affected by an unfavourable introduction of force by the driver in the same direction. Also, if it is asymmetrically deformed, the staple may jam in the region of the deformation tooling so that the staple can not readily be released.
The material of the staple is usually steel wire. When it is deformed or implanted, a staple of such material is predominantly plastically deformed, although it is always also elastically deformed so that when the staple is released from the dies it reopens slightly and spreads apart. Such spring-back effect has the disadvantage that wound edge adaptation cannot be controlled.
It is proposed according to EP-A-284 345 to compensate for the spring-back effect in such a way that the staple, when being implanted, is also elastically deformed and prestressed against its actual closing direction, so that on release of the implanted staple, such prestressing acts in the closing direction of the staple and thus against the unavoidable spring-back effect. In order to achieve this, a concave depression may be provided in the anvil, and a convex pressure cross-piece, formed complementarily with the depression, provided on the driver. As the driver is lowered, the cross-piece presses the crown of the staple into the depression. The cross-piece is provided on the driver above its two lateral legs which press the legs of the staple into the tissue by correspondingly bending the staple. Because of this construction, when the driver is lowered, only the legs of the driver act initially on the staple and deform it, whilst the pressure cross-piece can only come into action thereafter, or at the earliest shortly before the end of the actual deformation of the staple.
Such an expedient cannot, therefore, prevent the crown of staple from arching with the disadvantages mentioned above, the arching of the staple only being eliminated when said cross-piece arrives at the crown of the staple and presses the crown into the depression. Since the staple has already substantially reached its planned deformation, staple material is drawn over the lateral edges of the anvil in the direction of the center of the depression, so that the staple, by reason of static frictions of different extents occurring at the points of contact of the staple with the anvil, deforms asymmetrically and is clamped between the dies. It is most doubtful that the crown of the staple can be sufficiently elastically prestressed since the staple has already been substantially deformed and firmly fixed between the dies.
The arching of the staple can be limited by means of a stop arranged above the anvil and rigidly connected therewith as disclosed in EP-A-142 225. The crown, which shifts upwards during the deformation of the staple, is inhibited from further arching by means of the stop. However, the staple, or in fact its crown, must be introduced from the side with sufficient play into the space between the anvil and the stop, so that lateral displacement, and thus asymmetrical deformation, of the staple on the anvil can not effectively be prevented, because the stop can not secure the staple crown on the anvil.